The utilization of threaded closures for use in packaging of carbonated beverages has become very popular. The popularity is due in part to the fact that the consumer can open the package by merely unscrewing the closure from the container. No "bottle opening" tool is needed. Another advantage is that the consumer is able to remove the closure, dispense part of the contents from the container and reclose the container by merely screwing the closure back thereon. Since the sealing system is generally of high fidelity, there will be little loss of carbonation and the remaining packaged product will be suitable for use at a later time.
Despite these advantages, the threaded container-closure package has potentially a serious problem, i.e., premature release of the closure from the container which can occur with great force. The premature release occurs as the user turns the closure to remove it from the container. As the closure is turned, it moves axially upwardly thus breaking the seal between the top of the closure and the top of the container. Upon loss of the seal, pressurized gas enters between the sidewall of the closure and the container tending to bulge the closure sidewall outwardly. As the sidewall of the closure bulges outwardly, the closure threads are pulled away from engagement with the container threads thereby making the connection between the container and closure tenuous at best. If the gas is under sufficient pressure, the closure will be released from the container since the container-closure thread engagement is insufficient to contain the pressurized gas. This release is oftentimes with great force, thereby presenting danger to the consumer.
One of the most popular threaded closures used in packaging carbonated products is the nearly ubiquitous metal cap. To aid in preventing premature release of this type of closure the art has suggested providing a vent slot through the container threads. The slot provides a path for the pressurized gas to vent to the atmosphere, thus preventing closure bulge. See U.S. Pat. No. 4,007,848. In U.s. Pat. No. 4,007,851, another venting method for metal closures is shown. The closure is constructed to have, at a point adjacent the intersection of the sidewall and the top wall, at least one vent through which the pressurized gas may pass. Another type of system, one which uses circumferential venting, is shown in U.S. Pat. No. 1,739,659. These systems, while they may work in theory, are not particularly desirable as either they require modification in the design of the container threaded neck portion, they have dirt trapping openings in the closure itself, or they do not provide a sufficient amount of venting.
These problems can be solved by the utilization of thermoplastic closures. Thermoplastic closures can be designed so that a vent groove is cut on the inside surface of the closure sidewall across the closure threads. See U.S. Pat. No. 3,888,347. The width of the vent groove and the number of vent grooves utilized can be varied to provide the necessary venting rate for the conditions expected. Further, with this type of system, there will be no dirt entrapping openings exposed to the outside of the closure. (The use of such a groove on a metal closure is not practical as the metal closures used in packaging carbonated beverages are almost all roll formed on the container from a blank.)
Desirable as it may be, the location of the vent slot in the closure presents problems itself. The use of the vent slot requires a recessed cut in the closure sidewall across the closure thread, with the result being that the closure sidewall is thinner at the vent slot and unsupported by a continuous thread. Upon tightening the closure to the container, the weakened sidewall will expand outwardly as, in its weakened configuration, it cannot support the forces applied on it by the engagement of the container and closure threads. Also, when the closure is loosened from its seal position, the pressurized gas can cause the weakened closure sidewall to expand. Both, the closure expansion realized upon tightening and the closure expansion caused by the pressurized gas, jeopardize the closure-container thread engagement. When the thread engagement is compromised to the extent that the pressure inside the closure cannot be held by the threads, then premature release of the closure occurs. Using a closure with thickened sidewalls is not an answer as such a closure uses more thermoplastic material per closure and could not compete economically in the marketplace.
Therefore, it is an object of this invention to provide a thermoplastic closure having a thin structurally sound sidewall while at the same time having a vent groove in the inside surface of the closure sidewall and extending across the closure threads.